The Collaborative International Dictionary of English v.0.48:

Indentation \In`den*ta"tion\, n.
   1. The act of indenting or state of being indented.
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   2. A notch or recess, in the margin or border of anything;
      as, the indentations of a leaf, of the coast, etc.
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   3. A recess or sharp depression in any surface.
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   4. (Print.)
      (a) The act of beginning a line or series of lines at a
          little distance within the flush line of the column or
          page, as in the common way of beginning the first line
          of a paragraph.
      (b) The measure of the distance; as, an indentation of one
          em, or of two ems.
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   Hanging indentation, or Reverse indentation, indentation
      of all the lines of a paragraph except the first, which is
      a full line; also called a hanging indent.
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